A solid electrolytic capacitor is a device which comprises a metal foil subjected to etching treatment and having a large specific surface area (anode substrate) on which is formed an oxide dielectric layer and a solid semiconductor layer (hereinafter simply referred to as a solid electrolyte) as an opposing electrode outside the oxide dielectric layer and preferably further an electric conductor layer such as an electrically conducting paste. The device has an cathode lead terminal connected to the metal foil and an anode terminal connected to the electric conductor layer and as a whole is completely sealed by an epoxy resin or the like and is put into use as a capacitor part in electric products over a wide range.
As the method for forming a solid electrolyte layer, there have conventionally been known a method of fusing a solid electrolyte onto a dielectric layer which has been formed on a metal surface and has a porous or fine void structure to form a solid electrolyte layer on the dielectric layer and a method by producing a solid electrolyte on a dielectric layer.
According to recent digitalization of electronic equipment, an increase in the operation speed of personal computers, a compact capacitor having a high capacity and a low impedance in high-frequency regions is being demanded. Hitherto, as a capacitor having a high capacity, there have been known electrolytic capacitors such as an aluminum electrolytic capacitor and a tantalum electrolytic capacitor. However, an aluminum electrolytic capacitor has problems that it has a high impedance in high frequency regions because it uses an ion conducting liquid electrolyte and also has a poor temperature characteristic. A tantalum capacitor, which uses manganese oxide as the electrolyte, has a problem that it has a high impedance in high frequency regions since the manganese oxide has a relatively high specific resistance.
To cope with the problems, it has been heretofore proposed to use an electrically conducting polymer having an electron conductivity as the solid electrolyte. For example, there has been known use of polymers such as an intrinsic electrically conducting polymer having an electric conductivity of 10−3 to 103 S/cm [JP-A-1-169914 (the term “JP-A” as used herein means laid-open publication of unexamined Japanese patent application) (U.S. Pat. No. 4,803,596)], polyaniline (JP-A-61-239617), polypyrrole (JP-A-61-240625), polythiophene derivative (JP-A-2-15611 (U.S. Pat. No. 4,910,645)), and polyisothianaphthene (JP-A-62-118511). Many of these electrically conducting polymers comprising a π-conjugated system are used as a composition containing a dopant. Further, recently, not only dopants are used singly but also they are used in combination with manganese dioxide (JP-B-6-101418 (the term “JP-B” as used herein means publication of examined Japanese patent application) (U.S. Pat. No. 4,959,753)) or a filler (JP-A-9-320901).
Thus, the reason why electrically conducting polymers are drawing an attention as the solid electrolyte is that they may be improved to have a sufficiently high electric conductivity. However, there is a problem in that if the electric conductivity is higher than a proper range, the leakage current greatly increases to cause short circuit, whereas if it is lower than the proper range, the frequency properties are deteriorated to cause a large reduction in the capacity. Accordingly, it is a subject of development how to control the electric conductivity of the solid electrolyte in a proper range and attain the heat resistance and thermal stability thereof.
On the other hand, as the method for forming a solid electrolyte layer, there have hitherto been known a method of forming by fusion a solid electrolyte layer on a dielectric layer of a valve acting metal surface having a microfine porous or void structure and a method of producing the above-mentioned electrically conducting polymer on a dielectric layer. More specifically, for example, in the case of using a polymer of a 5-membered heterocyclic compound such as pyrrole or thiophene, there have been known a method where an anode foil is dipped in a solution of a 5-membered heterocyclic compound in a lower alcohol/water, and then dipped in an aqueous solution having dissolved therein an oxidizing agent and an electrolyte to give rise to chemical polymerization, thereby forming an electrically conducting compound (JP-A-5-175082), and a method where a 3,4-dioxyethylenethiophene monomer and an oxidizing agent each preferably in the form of a solution are applied separately differing in time or simultaneously on an oxide cover layer of a metal foil to thereby form a solid electrolyte layer (JP-A-2-15611 (U.S. Pat. No. 4,910,645) and JP-A-10-32145 (European Patent Application Laid-open No. 820076(A2)). In particular, JP-A-10-32145 (European Patent Application Laid-open No. 820076(A2)) discloses polymers of a monomer selected from pyrrole, thiophene, furan, aniline and derivatives thereof and doped with an aromatic polysulfonic acid having a plurality of sulfonic acid groups in the molecular structure and also discloses as a production method, a polymerization method in which a monomer is introduced after a mixed solution of the above-mentioned monomer and an oxidizing agent is coated and dried or after an oxidizing agent is introduced. Also, JP-A-10-32145 (European Patent Application Laid-open No. 820076(A2)) discloses a production method in which the dopant of the aromatic polysulfonic acid is utilized as a constituent component of the oxidizing agent (ferric salt) and describes that the solid electrolytic capacitor provided therewith has an advantage that it is excellent in high temperature resistance and prevention of deterioration of static capacity. As the oxidizing agent used in the prior art in the case of chemical polymerization of 5-membered aromatic cyclic compounds, for example, thiophene, there have been known iron (III) chloride, Fe(ClO4)3, organic acid iron (III) salt, inorganic acid iron (III) salt, alkyl persulfate, ammonium persulfate (hereafter, abbreviated as APS), hydrogen peroxide, K2Cr2O7, etc. (JP-A-2-15611 (U.S. Pat. No. 4,910,645)), cupric compounds, silver compounds, etc. (JP-A-10-32145 (European Patent Application Laid-open No. 820076(A2)).
More specifically, the capacitor comprising a solid electrolyte of the manganese dioxide is disadvantageous in that the oxide layer is ruptured at the thermal decomposition of manganese nitrate and the impedance property is unsatisfactory. Use of lead dioxide must be accompanied with a consideration on the environment.
The solid electrolytic capacitor using a tetracyanoquinodimethane (TCNQ) complex salt has good heat fusion workability and excellent electric conductivity but the TCNQ complex salt itself is said to have a problem in the heat resistance and in turn, a poor reliability in the soldering heat resistance.
In order to overcome these problems, the above-mentioned electrically conducting polymer such as polypyrrole is applied to the solid electrolyte on a dielectric surface by electrochemical polymerization or chemical polymerization but the conventional capacitors with electrically conducting polymer such as polypyrrole has a problem that their capacitor characteristics vary greatly depending on the humidity resistance load.
Further, as associated with humidity resistance load, heat resistance is highly demanded. For example, soldering heat resistance (reflow characteristic) when a capacitor element is molded into a capacitor component is laid importance so that a capacitor element having a high heat resistance is demanded.
The electrically conducting polymer layer as a solid electrolyte must have a high conductivity and be a heat resistant material which can be formed so as to cover all the inner surfaces of the pore cavities inside the anode and endure the above-mentioned soldering temperature. In addition, the following conditions are required therefor.
That is, firstly, it can relax thermal stress generated by soldering, etc., secondly it has good adhesion both mechanically and electrically with an electrically conducting paste layer formed on the electrically conducting polymer layer, and thirdly it has a good ability of repairing an oxide dielectric film when it is conducted electrically.
With respect to the first condition of relaxation of thermal stress, it has been proposed to form an electrically conducting polymer layer having a certain thickness on the outer surface of anode on which an oxide dielectric film has been formed. To achieve this, there have been disclosed a method in which a first electrically conducting polymer layer which serves as a precoat layer is formed by chemical polymerization and then a second electrically conducting polymer layer is formed by electrolytic polymerization using the first polymer layer as an electrode (JP-A-63-173313 (U.S. Pat. No. 4,780,796) and a method in which a solution of electrically conducting polymer containing a filler is coated to form an electrically conducting polymer layer (JP-A-9-320901). The ability of relaxing thermal stress is influenced not only by the thickness of the layer but also by the structure of the layer. As the electrically conducting polymer having a different macroscopic structure, there has been disclosed a sponge-like electrically conducting polymer molded article containing the electrically conducting polymer as a continuous phase (JP-A-8-53566).
With respect to the improvement in the adhesion between the second electrically conducting polymer layer and the electrically conducting paste layer, it has been proposed to form unevenness on the surface of the electrically conducting polymer layer. To achieve this, there have been disclosed a method in which a solution having mixed therein microfine powder is coated as it is on a first electrically conducting polymer layer to provide the microfine powder and then a second electrically conducting polymer layer is formed thereon (JP-A-7-94368 (U.S. Pat. No. 5,473,503)) and a method in which microfine electrically conducting polymer powder is injected or sprayed as contained in a gas or liquid flow on a first electrically conducting polymer layer and then a second electrically conducting polymer layer is formed thereon (JP-A-9-320898 (European Patent Application Laid-open No. 825626(A2)).
With respect to the improvement in the ability of repairing oxide dielectric film as the third point, a method has been disclosed in which a tantalum solid electrolytic capacitor has a structure that has an electrically conducting polymer compound covers an oxide dielectric film so that cavities remain in the pores and is provided with an oxygen source to enable insulation of the electrically conducting polymer compound upon electric conduction (JP-A-7-122464 (U.S. Pat. No. 5,455,736)).
As stated above, the conventional electrically conducting polymers are insufficient in heat resistance.
Also, the method proposed for the relaxation of thermal stress is disadvantageous in that the electrically conducting polymer layer formed by electrolytic polymerization has less surface unevenness and the adhesion with the electrically conducting paste layer is poor. In the method of forming an electrically conducting polymer layer having a certain thickness by coating an electrically conducting polymer solution containing a filler (JP-A-9-320901), the ability of relaxing thermal stress per thickness of the polymer layer is low so that the polymer layer has to have a relatively large thickness, which is disadvantageous for the down-sizing and increasing capacity of the device.
The sponge-like electrically conducting polymer molded article (JP-A-8-53566) is not applied to a solid electrolytic capacitor and the production method for the sponge-like electrically conducting polymer molded article according to the invention described in this prior art is a method in which an electrically conducting polymer solution is cooled to freeze the solvent and carry out polymerization and thereafter the solvent is removed by freeze-drying or thawing and hence its operation is cumbersome and further the oxide dielectric film tends to suffer damages upon freezing and thawing, so that it cannot be said to be a method which is applicable to solid electrolytic capacitors.
Next, with respect to the proposal for improving the adhesion with the electrically conducting paste layer by forming unevenness on the surface of the electrically conducting polymer layer, the method in which a solution having mixed therein microfine powder is coated as it is on a first electrically conducting polymer layer to provide the microfine powder (JP-A-7-94368 (U.S. Pat. No. 5,473,503)) has a problem that the conditions of forming unevenness fluctuate in a device or lot or between lots. Also, the method in which microfine powder of an electrically conducting polymer is injected or sprayed as contained in a gas or liquid flow on a first electrically conducting polymer layer (JP-A-9-320898 (European Patent Application Laid-open No. 825626(A2)) has a problem that the ability of relaxing thermal stress per thickness of the polymer layer is low so that the polymer layer is required to have a relatively large thickness, which is disadvantageous for the down-sizing and increasing capacity of the device.
Further, a proposal has been made to improve the ability of repairing the oxide dielectric film. In the method in which an electrically conducting polymer compound covers the oxide dielectric film with leaving cavities in the pores (JP-A-7-122464 (U.S. Pat. No. 5,455,736)), the ratio of cavities in the pores is adjusted by repetition of oxidative polymerization so that when formation of a thick layer of electrically conducting polymer on the outer surface of the anode is contemplated the cavities already existing in the pores tend to be closed. Therefore, there is a problem that the formation of an electrically conducting polymer layer having a certain thickness on the outer surface and securing cavities in the pores are not fulfilled simultaneously. Also there is a problem that failure of forming unevenness on the surface of the polymer layer results in a poor adhesion with the electrically conducting paste layer.
As described above, upon the production of a capacitor, further improvements are required on the material for the solid electrolyte, production method thereof, heat stability, homogeneity of the film and the like.
Under these circumstances, an object of the present invention is to provide a solid electrolytic capacitor having excellent heat resistance comprising an electrically conducting polymer layer excellent in the ability of relaxing thermal stress, adhesion with an electrically conducting paste layer, ability of repairing an oxide dielectric film.
Also, an object of the present invention is to provide a capacitor satisfying the requirements with respect to the reduction in the weight, high capacity, high frequency property, tan δ, leakage current, heat resistance (reflow property), durability, etc.
Further, an object of the present invention is to provide a method for the production of an electrolytic capacitor having the above-mentioned properties and thereby providing a solid electrolytic capacitor which is excellent in not only initial properties such as loss factor, leakage current, heat resistance, equivalent series resistance and impedance in high frequency regions but also durability in a sparking voltage test, long-term reliability (durability under high temperature and high humidity conditions, etc.).